Key Clicks and Splatter: Keeping Your Signal Clean on the Air
Every operator has tuned across a band and heard the signal that seems to be everywhere: a loud station whose sharp clicks or ragged sidebands spread over a dozen channels on either side. The operator is usually unaware of it. Nobody reports a problem because the stations affected are busy being annoyed. This guide explains how key clicks and splatter come about, why they matter to everyone on the band, how to check your own signal and what to adjust when it is not clean. It applies to CW, to single sideband and to digital modes, and it does not need test equipment beyond what most modern stations already have.
What Key Clicks and Splatter Are
A transmitted signal occupies a certain bandwidth. A well-formed CW signal takes up only a few hundred hertz, and a good SSB signal takes about 2.4 to 2.8 kilohertz. When something distorts the signal, energy appears outside that bandwidth. On CW the distortion usually comes from keying that is too abrupt, which produces sharp clicks that you can hear well away from the carrier. On SSB the distortion comes from overdriving a stage, which produces intermodulation products, heard as splatter, on both sides of the signal. The physics behind both is the same. A sharp edge in time means a wide spread in frequency, and a nonlinear stage creates products at frequencies that were never meant to be there. The article on key clicks gives the general background, and the page on intermodulation describes how products arise.
Splatter and clicks matter for three reasons. They cause interference to other stations that have done nothing wrong. They can break the rules, because every national regulation requires emissions to stay within the band and within sensible limits of purity. And they reduce your own effectiveness, because a station with a wide, dirty signal is often turned away by the other operator who is trying to avoid the noise. In a contest, where stations sit a few hundred hertz apart, a dirty signal makes enemies fast.
Key Clicks on CW
When you press a key, the transmitter output does not jump instantly to full power. The leading edge rises over a short time called the rise time, and the trailing edge falls over the fall time. If these times are very short, the envelope of the signal has sharp corners, and sharp corners contain energy at many frequencies. If the times are longer, the envelope is a smooth shape, the spectrum is narrow and the clicks vanish. Most modern transceivers let you set the rise time, often in steps such as 2, 4, 6 and 8 milliseconds.
A common guideline is a rise and fall time of about 5 milliseconds for ordinary CW speeds. Faster speeds can tolerate slightly shorter times and very slow sending can use longer ones, but going much below 2 or 3 milliseconds generally makes the signal wider than necessary, and going far above 8 to 10 milliseconds makes dots sound soft and mushy at higher speeds. The aim is to be as short as you can without producing audible clicks off frequency.
Where Clicks Come From
- Rise time set too short. This is the first thing to check in the transceiver’s menu.
- Keying a stage that is not shaped. A home-built transmitter or an external amplifier may be keyed with a hard on and off, with no shaping network.
- Amplifier relays and QSK. A relay that closes late can clip the first dot and produce a click. A relay that opens early can chop the end of a character.
- Power supply sag. A weak supply or a thin cable can let the voltage fall at the moment the key goes down, giving a chirp or a sharp edge.
- Overdriven stages. If the exciter drives the amplifier past its linear range, the keyed waveform is clipped, and clicks appear even if the rise time is correct.
The paddle and keyer setup does not change the shape of the keying envelope, but it does determine the timing. If you are tuning the whole CW chain, the guide to iambic keying modes and keyer settings is a useful companion.
Splatter on SSB
On sideband the issue is overdrive. The microphone gain, the speech processor and the drive to the final amplifier all add up, and when a stage reaches its limit the peaks are flattened. Flattened peaks produce harmonics and intermodulation products, which land on adjacent frequencies. In practice, this happens when the microphone gain is set too high and the compression too heavy, when the ALC meter is pinned against its limit or when an external amplifier receives more drive than it can handle.
The ALC is the first line of defence, and it is often misunderstood. It is meant to touch the speech peaks only lightly, not to flatten every syllable. Many operators watch the power meter and turn up the gain until they see a high reading, which makes the ALC work hard and distorts the audio. A clean signal is usually a little quieter than a hot one, but it is far more readable and does not spread. Our guide to SSB audio settings for microphone gain, compression and ALC walks through a safe adjustment sequence.
The Linear Amplifier
An amplifier is a common source of splatter because it is easy to overdrive. The rule is to supply only the drive that the amplifier needs for its rated output, and to watch for flat-topping, where the output power stops growing while the drive increases. The article on choosing an HF linear amplifier covers drive levels, and the same lesson applies to every amplifier: more drive is not more signal once the stage is saturated. If the amplifier is a solid-state design with a built-in ALC line, check that the line is connected to the transceiver and set correctly.
How to Check Your Own Signal
You cannot judge your signal by listening on the same receiver, because the receiver is next to your transmitter and is overloaded by it. You need an independent view. There are several ways to get one.
- Ask for a report. A friend a few kilometres away, tuned 2 or 3 kilohertz off your frequency, can tell you what he hears. Ask him to listen above and below your signal.
- Use a second receiver. A small SDR with a separate antenna lets you see your own signal on a waterfall, provided that you reduce the gain so that the SDR is not overloaded. Our piece on panadapters and spectrum scopes explains how to read the display.
- Use a monitor scope. A dedicated scope that samples the transmitter output shows the envelope on a screen, so you can see flattening of peaks or sharp rise edges.
- Use a remote receiver. A public web SDR near you can show your signal from outside your own station. Choose one that is far enough away not to be overloaded.
- Use the built-in scope. Many current transceivers have a spectrum display. It is useful for relative comparisons, but its dynamic range is limited, so do not rely on it for fine detail.
Whichever method you use, make a test on a quiet frequency, at a time when you will not disturb anyone, and use your normal power and settings. Change one adjustment at a time and observe the result, so you learn what each setting does. Record the settings that gave a clean signal so you can return to them after a firmware update or a reset.
Symptoms, Causes and Fixes
The table lists the most common problems and the first step to take for each.
| Symptom | Likely cause | First fix |
|---|---|---|
| Sharp clicks heard well off frequency on CW | Rise and fall time too short | Increase the CW rise time in the menu to about 5 ms |
| Dots sound soft and weak | Rise time too long for the speed | Reduce the rise time by one step |
| Chirp at the start of each element | Supply sag or oscillator pulling | Check the power cable and supply regulation |
| Splatter on both sides of an SSB signal | Microphone gain or compression too high | Lower the gain until the ALC just touches |
| Wide signal only when the amplifier is on | Amplifier overdriven | Reduce the drive to the rated level |
| Wide digital signal | Audio level too high into a sound interface | Lower the audio level until the ALC shows no action |
Digital Modes Are Not Immune
A digital mode such as FT8 transmits a steady tone with constant amplitude, so it should not produce key clicks, but it can splatter all the same. The cause is the audio level sent from the computer. If the level is too high, the transceiver’s modulator or the amplifier distorts the tones, and the result is a row of spurious products on either side. The usual symptom is a signal that looks wide on a waterfall and attracts reports of splatter. The cure is to lower the audio drive until the ALC does not move at all, and to reduce the power rather than raise the level. The setup tips in our guide to receiver dynamic range and intermodulation are useful here, because they show what it looks like when a strong signal overloads a stage on the receiving side as well.
Good Habits for a Clean Station
Cleanliness is a habit more than a one-time adjustment. Check your signal again whenever you change something in the chain: a new microphone, a different amplifier, a new computer interface, a firmware update or a change of power supply. Each of these can alter levels and timing without any obvious sign. Keep a short note of the settings that produced a clean signal, including the CW rise time, the microphone gain, the compression level and the audio level for digital modes. When you work a contest or a pileup, where it is tempting to push more power and more compression, remember that a station that is easy to copy beats one that is merely loud. A narrow, clean signal at moderate power is heard by the people you want to reach, and it leaves the neighbouring channels free for the stations who share the band with you.
Common Mistakes
- Judging your signal by the sound in your own headphones. The receiver is overloaded and tells you nothing about what others hear.
- Adjusting several things at once. You will not know which change improved or worsened the signal.
- Setting the rise time to the minimum for the sake of a snappier sound. The signal will be wider and noisier.
- Running the ALC deep into its range. The meter is a warning, not a target.
- Trusting a spectrum display on a transceiver for fine detail. Use it for comparison and use an independent receiver for a final check.
- Ignoring reports. If another operator tells you about clicks or splatter, test it. They are usually right.
The Bottom Line
A clean signal is a courtesy to every other operator on the band, and it is also a rule of the service. For CW, set the rise and fall time to about 5 milliseconds, check that external keyed stages are shaped and make sure the power supply does not sag. For sideband and digital modes, keep the microphone gain and audio level low enough that the ALC barely moves, and give an amplifier only the drive it needs. Check your signal with an independent receiver or an observer, change one setting at a time and note the result. Ten minutes of testing on a quiet frequency can make your signal narrower, easier to copy and easier to live with, and it will save you from the reports that nobody likes to receive.
Frequently Asked Questions
What causes key clicks on CW?
Keying that turns the signal on and off too abruptly. A short rise or fall time creates sharp edges that spread energy over a wide range of frequencies, which is heard as clicks off frequency.
What rise time should I use for CW?
About 5 milliseconds is a common starting point for ordinary speeds. Adjust it by listening on a second receiver or by asking for a report, and choose the shortest time that is free of clicks.
What is splatter?
Splatter is distortion that spreads energy onto adjacent frequencies, usually from overdriving the microphone stage, the speech processor or an amplifier on sideband.
How can I check my signal without special equipment?
Ask a nearby operator to listen a few kilohertz either side of your frequency, or use an SDR with a separate antenna and reduced gain to look at your signal on a waterfall.
Can FT8 produce splatter?
Yes, if the audio level is set too high. The tones become distorted and spurious products appear. Reduce the audio drive until the ALC shows no action and the signal looks narrow.
Does my amplifier make my signal dirty?
It can if it is overdriven or if its keying and bias are not shaped properly. Supply only the rated drive, connect the ALC line if the amplifier has one and check the signal with the amplifier on and off.